Method for growing thin films

ABSTRACT

The invention relates to a method and apparatus for growing a thin film onto a substrate, in which method a substrate placed in a reaction space ( 21 ) is subjected to alternately repeated surface reactions of at least two vapor-phase reactants for the purpose of forming a thin film. According to the method, said reactants are fed in the form of vapor-phase pulses repeatedly and alternately, each reactant separately from its own source, into said reaction space ( 21 ), and said vapor-phase reactants are brought to react with the surface of the substrate for the purpose of forming a solid-state thin film compound on said substrate. According to the invention, the gas volume of said reaction space is evacuated by means of a vacuum pump essentially totally between two successive vapor-phase reactant pulses. By virtue of transporting the different starting material species at different times through the apparatus effectively isolates the starting materials from each other thus preventing their premature mutual reactions.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/949,688, filed Dec. 3, 2007, issued Mar. 3, 2009 as U.S. Pat. No.7,498,059, which is a continuation of U.S. application Ser. No.09/855,321, filed May 14, 2001, issued Jul. 29, 2008 as U.S. Pat. No.7,404,984, which is a continuation of U.S. application Ser. No.09/482,625, filed Jan. 14, 2000, issued Jun. 3, 2003 as U.S. Pat. No.6,572,705, which is a continuation of U.S. application Ser. No.08/682,705, filed Sep. 25, 1996, issued Jan. 18, 2000 as U.S. Pat. No.6,015,590, which is the National Phase U.S. application of PCTapplication No. PCT/FI95/00658, claiming the priority benefit under 35U.S.C. §119 from Finnish Application No. 945611, filed Nov. 28, 1994.

FIELD OF THE INVENTION

The present invention relates to a method for growing thin films onsubstrates in a reaction space by alternate repeated reactions of atleast two vapor phase reactants with the substrates.

In the present method, the substrate is typically located in a reactionspace, wherein it in accordance with the Atomic Layer Epitaxy (ALE)method is subjected to alternately repeated surface reactions of atleast two different reactants. According to the present method, thereactants are admitted repetitively and alternately each reactant at atime from its own source in the form of vapor-phase pulses into thereaction space. Here, the vapor-phase reactants are allowed to reactwith the substrate surface for the purpose of forming a solid-state thinfilm on the substrate.

While the method is most appropriately suited for producing so-calledcompound thin films using as the reactants such starting materials thatcontain component elements of the desired compound thin-film, it mayalso be applied to growing elemental thin films. Of compound filmstypically used in the art, reference can be made to ZnS films employedin electroluminescent displays, whereby such films are grown on a glasssubstrate using zinc sulfide and hydrogen sulfide as the reactants inthe growth process. Of elemental thin films, reference can be made tosilicon thin films.

The invention also concerns an apparatus suited for producing thinfilms, comprising a reaction chamber with gas flow channels suited foran inflow of vapor phase reactant pulses and an outflow of reactionproducts, wherein at least a portion of the gas flow channels have anarrow, oblong cross-section for minimizing the volume of the reactionspace.

The apparatus comprises a reaction space into which the substrate can beplaced, and at least two reactant sources from which the reactants usedin the thin-film growth process can be fed in the form of vapor-phasepulses into the reaction space. The sources are connected to thereaction space via reactant inflow channels, and outflow channels areconnected to the reaction space for removing the gaseous reactionproducts of the thin-film growth process as well as the excess reactantsin vapor phase.

BACKGROUND AND SUMMARY OF THE INVENTION

Conventionally, thin-films are grown using vacuum evaporationdeposition, the Molecular Beam Epitaxy (MBE) and other vacuum depositionmethods, different variants of the Chemical Vapor Deposition (CVD)method, including low-pressure and metal-organic CVD and plasma-enhancedCVD, or alternatively, the above-described deposition method ofalternately repeated surface reactions called the Atomic Layer Epitaxy(ALE) method. In the MBE and CVD methods, besides other processvariables, the thin-film growth rate is also affected by theconcentrations of the starting material inflows. To achieve a uniformthickness of the layers deposited by the first category of conventionalmethods, the concentrations and reactivities of starting materials musthence be carefully kept constant all over the substrate area. If thestarting materials are allowed to mix with each other prior to reachingthe substrate surface as is the case in the CVD method, for instance, achance of their premature mutual reaction arises. Then, the risk ofmicroparticle formation already within the inflow channels of thegaseous reactants is imminent. Such microparticles have a deterioratingeffect on the quality of the thin film growth. Therefore, thepossibility of premature reactions in MBE and CVD reactors is avoided byheating the starting materials no earlier than at the substratesurfaces. In addition to heating, the desired reaction can be initiatedusing, e.g., a plasma or other similar activating means.

In the MBE and CVD processes, the growth of thin films is primarilyadjusted by controlling the inflow rates of starting materials impingingon the substrate. By contrast, the ALE process is based on allowing thesubstrate surface qualities, rather than the starting materialconcentrations or flow variables, to control the deposition rate. Theonly prerequisite in the ALE process is that the starting material isavailable in sufficient concentration for thin-film formation on allsides of the substrate.

The ALE method is described in the FI patent publications 52,359 and57,975 and in the U.S. Pat. Nos. 4,058,430 and 4,389,973, in which alsosome apparatus embodiments suited to implement this method aredisclosed. Apparatus constructions for growing thin films are also to befound in the following publications: Material Science Reports 4(7)(1989), p. 261, and Tyhjiötekniikka (Finnish publication for vacuumtechniques), ISBN 951-794-422-5, pp. 253-261.

In the ALE growth method, atoms or molecules are arranged to sweep overthe substrates, thus continuously impinging on their surface so that afully saturated molecular layer is formed thereon. According to theconventional techniques known from the FI patent publication No. 57,975,the saturation step is followed by an inert gas pulse forming adiffusion barrier which sweeps away the excess starting material and thegaseous reaction products from above the substrate. The successivepulses of different starting materials and of diffusion barriers of aninert gas separating the former accomplish the growth of the thin filmat a rate controlled by the surface chemistry properties of thedifferent materials. Such a reactor is called a “traveling-wave”reactor. For the function of the process it is irrelevant whether thegases or the substrates are moved, but rather, it is imperative that thedifferent starting materials of the successive reaction steps areseparated from each other and arranged to impinge on the substratesuccessively.

Most vacuum evaporators operate on the so-called “single-shot”principle. Herein, a vaporized atom or molecule species can impinge onthe substrate only once. If no reaction of the species with thesubstrate surface occurs, the species is bounced or re-vaporized so asto hit the apparatus walls or the inlet to the vacuum pump undergoingcondensation therein. In hot-wall reactors, an atom or molecule speciesimpinging on the reactor wall or the substrate may become re-vaporized,whereby advantageous conditions are created for repeated impingements ofthe species on the substrate. When applied to ALE reactors, this“multi-shot” principle can provide, i.e., improved material utilizationefficiency.

In conventional ALE apparatuses, a characterizing property is that thedifferent starting materials of the reaction are understood to beisolated from each other by means of a diffusion wall formed by an inertgas zone traveling between two successive pulses of starting materials,cf. above-cited FI patent publication No. 57,975 and the correspondingU.S. Pat. No. 4,389,973. The length of the inert gas zone acting as thedownstream flowing diffusion wall is such that only approx. onemillionth of the reactant gas molecules have a sufficient diffusionvelocity to travel under the prevailing conditions in the counterflowdirection to a distance greater than the thickness of the isolatingdiffusion wall employed in the method.

However, notwithstanding the high reliability of the above-describedarrangement, it has some disadvantages. For instance, the cross sectionsand shapes of piping in practical reactor constructions vary between,e.g., the infeed manifold and the substrates, whereby the thickness andshape of the diffusion wall become difficult to control and the startingmaterials may become carried over into contact with each other. Thediffusion wall may also become destroyed in the nozzles feeding thevapor-phase reactant to the substrates, in gas mixers or at otherdiscontinuity points of the piping. The laminarity of gas inflow mayalso become disturbed by a too tight bend in the piping.

Intermixing of starting materials in flow systems cannot be preventedsimply by keeping the gas volumes apart from each other, because mixingmay also occur due to adherence of molecules from a starting materialpulse on the apparatus walls or discontinuities thereof, wherefrom themolecules may then gain access with the molecules of the successivestarting material pulse.

It is an object of the present invention to overcome the drawbacks ofconventional technology and to provide an entirely novel arrangement forgrowing thin films.

The goal of the invention is achieved by virtue of admitting vapor-phasepulses of the starting material reactants into the ALE reactor so thateach starting material pulse is individually driven through the pipingand reaction space of the apparatus isolated from the other pulses.According to the invention, this concept is implemented by means ofpurging the gas volume of the reaction space containing reactive gasbetween two successive vapor-phase pulses essentially entirely whichmeans a purging efficiency of at least 99%, advantageously 99.99%.Thence, all the reacting gas, which in practice refers to the entire gasvolume filled with the vapor-phase reactant, is purged from the reactionspace between the successive pulses. The reactant pulses of differentstarting materials will thus remain isolated from each other, whereby nomixing of the reactants can occur.

In the apparatus suited to implement the method, the outflow channelsare connected to a pump capable of evacuating the reaction space to avacuum, whereby the pump capacity is dimensioned sufficiently high topermit full evacuation of a volumetric amount of gas corresponding tothe gas volume of the reaction space out from the reaction space duringthe interval between two successive vapor-phase reactant pulses.Accordingly, the pump must have a volumetric flow capacity per timeunit, advantageously over the interval between two successivevapor-phase reactant pulses, that is greater than the gas volume of thereaction space.

More specifically, the method according to the invention is principallycharacterized by a reaction space in which the gas volume is evacuatedessentially totally between two successive vapor phase reactant pulses.

Furthermore, the apparatus according to the invention is principallycharacterized by a reaction chamber with gas flow channels suited for aninflow of vapor phase reactant pulses and an outflow of reactionproducts, wherein at least a portion of the gas flow channels have anarrow, oblong cross-section for minimizing the volume of the reactionspace.

In the context of the present invention, the term “evacuation” is usedgenerally referring to the removal of reactant residues in the vaporphase. The evacuation of the reaction space can be accomplished bypurging the gas volume of the apparatus by means of at least one pumpingcycle capable of lowering the internal pressure in the apparatus to asufficiently high vacuum. When required, the apparatus may besimultaneously filled with an inactive gas which promotes the purging ofthe reactant residues from the reaction space.

In the present context, the term “inactive” gas is used to refer to agas which is admitted into the reaction space and is capable ofpreventing undesired reactions related to the reactants and thesubstrate, respectively. Such reactions include the reactions of thereactants and the substrate with possible impurities, for instance. Theinactive gas also serves for preventing reactions between the substancesof the different reactant groups in, e.g., the inflow piping. In themethod according to the invention, the inactive gas is also usedadvantageously as the carrier gas of the vapor-phase pulses of thereactants. According to a preferred embodiment, in which the reactantsof the different reactant groups are admitted via separate inletmanifolds into the reaction space, the vapor-phase reactant pulse isadmitted from one inflow channel while the inactive gas is admitted fromanother inflow channel, thus preventing admitted reactant from enteringthe inflow channel of another reactant. Of inactive gases suited for usein the method, reference can be made to inert gases such as nitrogen gasand noble gases, e.g., argon. The inactive gas may also be an inherentlyreactive gas such as hydrogen gas serving to prevent undesirablereactions, e.g., oxidization reactions, from occurring on the substratesurface.

According to the invention, the term “reaction space” includes both thespace in which the substrate is located and in which the vapor-phasereactants are allowed to react with the substrate in order to grow thinfilms, namely, the reaction chamber, as well as the gas inflow/outflowchannels communicating immediately with the reaction chamber, saidchannels serving for admitting the reactants into the reaction chamber,inflow channels or removing the gaseous reaction products of thethin-film growth process and excess reactants from the reaction chamber,outflow channels. According to the construction of the embodiment, thenumber of the inflow and outflow channels, respectively, can be variedfrom one upward. According to the invention, the reaction space is theentire volume to be evacuated between two successive vapor-phase pulses.

In the present context, the term “reactant” refers to a vaporizablematerial capable of reacting with the substrate surface. In the ALEmethod, reactants belonging in two different groups are conventionallyemployed. The reactants may be solids, liquids or gases. The term“metallic reactants” is used of metallic compounds which may even beelemental metals. Suitable metallic reactants are the halogenides ofmetals including chlorides and bromides, for instance, and metal-organiccompounds such as the thd complex compounds. As examples of metallicreactants may be mentioned Zn, ZnCl₂, TiCl₄, Ca(thd)₂, (CH₃)₃Al andCp₂Mg. The term “nonmetallic reactants” is used for compounds andelements capable of reacting with metallic compounds. The latter groupis appropriately represented by water, sulfur, hydrogen sulfide andammonia.

Herein, the term “substrate surface” is used to denote that top surfaceof the substrate on which the vapor-phase reactant flowing into thereaction chamber first impinges. In practice, said surface during thefirst cycle of the thin-film growing process is constituted by thesurface of the substrate such as glass, for instance; during the secondcycle the surface is constituted by the layer comprising the solid-statereaction product which is deposited by the reaction between thereactants and is adhered to the substrate, etc.

As mentioned above, in a practical embodiment the vapor-phase reactantsare conventionally driven by a carrier gas flow into the reactionchamber and further through it. Therefore, the vapor-phase reactantobtained from a source is mixed with the inert gas flow at some point ofthe apparatus. In the present embodiment, the term “reaction space gasflow channels” also includes that section of the reactant inflow pipeswhich is located after the control valves of the inactive gas flow.

A characterizing property of the present invention is that the differentstarting materials are not allowed to flow simultaneously in the pipingor reactor, reaction space, but rather, the piping and reaction spaceare evacuated from the contents of the preceding vapor-phase pulse priorto the admission of the next vapor-phase pulse. Advantageously, theinterval between the successive pulses is kept so long as to permit theevacuation of the reaction space using at least a double or triplepurging gas volume during the interval between the pulses. To achievemaximally efficient evacuation of reactant residues, the reaction spaceis purged with an inactive gas during the interval between the reactantpulses and the total volume of gas evacuated from the reaction spaceduring the interval between the reactant pulses amounts to at least 2-10times the volume of the reaction space. A design target value of lessthan 1%, advantageously less than 1%, of residual components of thepreceding vapor-phase reactant pulse remaining at the infeed of the nextpulse can be set for the evacuation efficiency. Operation according tothe invention can easily reach a situation in which the reaction spaceis purged to less than 1 ppm of reactant residues from the precedingpulse.

According to the present method, evacuation is most advantageouslyimplemented by connecting the reaction space to a pump whose volumetriccapacity during the interval between two successive vapor-phase reactantpulses is appreciably greater than the gas volume of the reaction space.As the interval between two successive reactant gas pulses typically isin the order of 1 s, this requirement can be met by connecting such apump to the reaction space that has a capacity sufficient to evacuateduring said interval a volumetric amount of gas which is advantageouslyat least 2-3 times, and particularly advantageously 4-10 times thevolume of the reaction space.

The invention can be implemented using any suitable pump capable ofestablishing a sufficient vacuum in the reaction space and having asufficient capacity. Examples of suitable pump types are: rotary vacuumpump, Roots' blower and turbo pump.

To achieve efficient evacuation, the apparatus according to theinvention has a design characterized by minimized volumes and pipingcross sections implemented in a construction with minimized number ofseams. The piping layout aims to avoid any structures which coulddisturb the laminarity of the flow pattern or act as difficult-to-purgegas pockets.

In conventional equipment constructions the above-described goals aredifficult to achieve, since gas volumes in the equipment are relativelylarge in relation to the volume occupied by the products and the gasflow occur via complicated paths. Obviously, a mere diffusion wallcannot purge all gas pockets contained therein. The problems areaccentuated in equipment designed for simultaneous thin film growth onmultiple substrates.

The present invention discloses a plurality of special propertiescontributing to the minimization of apparatus gas volume and theformation tendency of spalling films (microparticles). Simultaneously,the invention provides a particularly advantageous embodiment of anapparatus suited for simultaneous deposition of thin films on two ormore substrates.

An advantageous approach to reduce equipment contamination is to feedeach reactant group via a separate inflow channel directly into thereaction chamber. Preferably, the reactant is allowed to become mixedwith a carrier gas flow entering from the inflow channel of anotherreactant group prior to contacting the reactant with the substrate. Thepurpose of such mixing is to homogenize the gas flow passing over thesubstrate.

The above-described embodiment is particularly well suited for thin-filmgrowth processes using at least two compound component reactants. Theexit ends of the inflow channels of the different reactant groups, laterin the text called the reactant “infeed openings”, are adapted to exitinto the reaction chamber, close to the substrates of the thin filmstructures. Between the infeed openings is herein disposed a bafflewhich prevents the reactant inflow from one infeed opening from enteringdirectly the infeed opening of another reactant belonging to a differentreactant group. To eliminate the risk of reactant contamination, acarrier gas flow is hereby particularly advantageously driven throughthat inflow channel or channels which is/are currently not used for theinfeed of a reactant. Preferably, the reactant infeed openings aredisposed on the opposite sides of the baffle and the reactant inflowsare directed perpendicularly against the baffle, whereby the gas flowcan be spread into an essentially planar flow producing a “flattened”flow pattern. The carrier gas flow and the vapor-phase reactant flowentering from the opposite directions, respectively, that are flattenedby hitting the baffle are combined prior to taking their mixed flow intocontact with the substrate. It has been found that the intermixing ofdifferent species by diffusion is extremely efficient between theflattened gas flows resulting in excellent uniformity of the gas flowtaken to the substrate.

According to an alternative embodiment, the gas volume of the apparatusis minimized by designing those gas flow channels which communicate withthe reaction chamber to have a narrow, oblong cross section in order tominimize the volume of the reaction space. Hence, the gas flow channelshave a “flat” shape capable of producing a similar flattened gas flowpattern as in the arrangement of the above-described embodiment.

Typically, the flat gas flow channel according to the invention has across section with a width, orthogonal to the flow direction of the gaspulse front, of approx. 1-100 times the channel height. Advantageously,the width-to-height ratio is approx. 5:1-50:1, typically approx.30:1-5:1.

In both of the above-described embodiments, the reaction chamberenclosing the substrate is particularly advantageously designed to havethe chamber walls disposed close to the substrate being processed. Theinner top wall of the chamber is advantageously aligned parallel to thesubstrate top surface. In fact as noted earlier, the inner top wall ofthe chamber may be formed by another substrate.

Minimization of the gas volume in the apparatus improves the utilizationefficiency of reactants as a single reaction space can be simultaneouslyused for growing thin film onto at least two substrates. According tothe invention, this arrangement can be implemented by placing thesubstrates in separate reaction chambers which are stacked vertically orhorizontally to provide a reaction chamber pack in which the chambershave common gas flow channels in order to minimize the total volume ofthe reaction space. The number of vertically or horizontally stackedreaction chambers may be from 2 to 100 and as each of the chambers canbe used for processing at least two substrates simultaneously, the totalnumber of thin-film surfaces being processed may be varied in the rangeof 2-200, for instance.

According to a particularly advantageous embodiment, the apparatusaccording to the invention comprises vertically or horizontally stackedplanar elements, whereby said elements have recesses/groovescorresponding to the reaction chambers and gas flow channels machined tothem and at least a number of said elements is mutually identical. Theedge areas of the planar elements are provided with round, oralternatively, oblong notches or openings extending through the planarelement and forming said gas flow channels of said reaction space whensaid planar elements are stacked vertically or horizontally in order toform a reaction chamber pack. The number of the round openings on thereactant inflow side is advantageously one per each reactant group,which in practice means two openings. The number of oblong openingsrequired on the outflow side is only one.

The center parts of the planar elements can be provided with areasrecessed from the plane of the element so that the recesses areconnected at their reactant inflow and outflow sides, respectively, tosaid notches or openings. The recessed areas form the reaction chamberof the reaction space, or a portion thereof. The flow connectionsbetween the recessed areas and the gas flow channels act as restrictionsto the gas flows. The recessed areas of the element may be made so deepas to extend through the entire thickness of the planar element leavingthe center of the element open. Advantageously, the inner edges of therecessed areas conform along at least two opposite sides of the recessto the edges of the substrates, thus permitting the location of thesubstrates in the recesses. When desired, the inner edges of therecesses can be provided with brackets serving to support the substrate.In the latter case the broad walls of the reaction chamber pack areformed by substrates placed into the center openings of the planarelements, whereby the substrates may be aligned so as to, e.g., have thesubstrate top sides facing each other.

The above-described apparatus construction details make it possible toreduce the weight of the reaction space and minimize the number ofcomponents in the system. By arranging the reaction space to comprisevertically or horizontally stacked reaction spaces, the length of thegas inflow and outflow channels, respectively, can be reduced. This isparticularly true for the latter case in which the substrates themselvesserve as the broad walls of the reaction chambers.

The invention provides significant benefits over prior-art ALE reactors.Accordingly, the pulsing concept of the starting materials based onnever having two or more different starting material species transportedin the system simultaneously effectively isolates the starting materialsfrom each other thus preventing their premature mutual reactions. Shouldsuch reactions occur in the gas phase, CVD thin film growth wouldresult, whereby the reactor deviates from the operating conditions ofthe ALE process and the reactor cannot be called an ALE reactor anymore. In fact, the CVD thin film growth condition in conventional ALEreactors often causes the formation of the detrimental microparticledust/spalling.

According to the invention, the risk of CVD thin film growth iseliminated thereby yielding true surface-controlled thin film growth,and thus, excellent ALE process qualities; in fact, the apparatusaccording to the invention realizes the separate reaction stepscharacterizing a true ALE process.

The minimized surface areas and volumes also minimize the amount ofextra thin film growth in the piping, whereby the rate of dust/spallformation and need for cleaning are reduced. The small gas volume withoptimized fluid dynamics speeds the through-flow of gases and improvesthe purging of gases participating in the reactions, which is evidencedas faster process rate and improved thin film quality.

The evacuation steps and possible complementing step of flushing with aninactive gas also contribute to the efficient removal of moleculesadsorbed on the inner walls of the system and thus lessen the tendencyof the molecules to react with the molecule species of the successivereactant pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be examined in greater detail withreference to the appended drawings in which

FIG. 1 is a longitudinally sectioned side view of a simplified structurefor the reaction chamber pack in a first embodiment according to theinvention, and

FIG. 2 is a longitudinally sectioned side view of a simplified structurefor the reaction space construction in a second embodiment according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the apparatus construction shown therein comprisesa reaction space, or a reaction chamber pack 1, assembled from mutuallyidentical, stacked planar elements 10, in which pack the gas flowchannels 7, 4 and reaction chambers 13 are formed by openings andnotches made to the planar elements. The apparatus is shown toincorporate four reaction chambers 13 having placed therein eightsubstrates 12 onto which thin films are grown using the ALE process.Reference numeral 3 denotes the connection of the reaction chamber packto a pipe communicating with the inlet of a pump. The connectioncommunicates with the outflow channel 4 of vapor-phase reaction productsand excess reactants, whereby the outflow channel acts as collectingmanifold for the outflows from the reaction chambers. Correspondingly,reference numeral 2 denotes the inflow opening for vapor-phasereactants, whereby said inflow opening further communicates with thereactant inflow channel 7.

The planar elements are provided with an encircling suction groove 5 forcollecting any gas leaks. The suction groove communicates with theoutflow channel 4. The purpose of the suction groove is to avoid theaccess of external contamination into the reaction space and to preventreactants from leaking outside the reaction space. Thence, the grooveacts as an isolating gas seal for the reaction space.

When stacking the planar elements, between each two superimposedelements is placed an intermediate plate 6 suited for controlling therestriction of the gas flow by setting the cross section of the inletslit 8 from the inflow channel 7 into the reaction chamber 13 and thecross section of the outlet slit 14, that is, gas flow restrictions,from the reaction chamber to the outflow channel 4.

The upper half of the uppermost reaction chamber acts as the top plate 9of the reaction chamber pack, and correspondingly, the lower half of thelowermost reaction chamber acts as the bottom plate 11, which is mountedonto a support base 17. Between said top plate and bottom plate arestacked three mutually identical planar elements 10. Each planar elementforms firstly in combination with the substrate 12 the wall between twoadjacently stacked reaction chambers, and secondly, in combination withthe intermediate plates 6 and other auxiliary plates, forms the inflowand outflow channels 7, 4. The number of the planar elements may bevaried in the range 0-100 pcs.

The inflow/outflow channels 7, 4 and the reaction chambers are shaped tohave longitudinally a narrow, oblong cross section to facilitate a“flattened” gas flow and minimize the volume of the reaction space.

In the embodiment illustrated in FIG. 1, the vapor-phase reactant pulsesof different reactant groups are fed alternately into the inflow channel7. Prior to the feed, the gas inflow pulses are homogenized with aninactive gas flow in the inflow channel 7 or before. In the inflowchannel, shown longitudinally sectioned in the diagram, the vapor-phasereactant pulse travels flattened into a planar shape which proceedsalong the channel with a defined leading front. The flow front has awidth equal to the that of the substrate, which is approx. 10-30 cm, forinstance, while the thickness of the front is approx. 1-3 cm.

The flow travelling in the inflow channel is distributed evenly betweenthe in-parallel stacked reaction chambers 13 by dimensioning thereaction space with its gas flow restrictions so that the flowconductance of the inflow channel 7 is much higher than the flowconductance via the reaction chambers 13. The flow path through eachreaction chamber must have a conductance which is equivalent (equal) tothat of the gas paths via the other chambers. Then the pressure and flowrate is balanced between the individual reaction chambers, and thence,the thin film growth rate is also equal in the separate chambers. Insidethe reaction chamber 13, the flow pattern is equalized by virtue of thenarrow suction slit 14 formed to the outflow end of the reactionchamber. The suction slit may be formed by either a single, contiguousslit or a plurality of small, parallel slits which in the exit directionof the flow is/are preceded by the large-volume reaction chamber 13having a larger relative flow conductance than that of the slit(s).Then, the gas flow tends to exit via the slit(s) in an equallydistributed pattern. In the reaction chamber 13 this is manifested as anequalized cross-directional pressure gradient of the leading edge of thepropagating gas pulse meaning an equalized propagating gas front. Intests (with reduced reactant dosing) the gas front has been found tohave an extremely straight contour.

Ensuring the equalized cross-directional outflow pattern of gas front isextremely important, because the gas molecules tend to travel toward thedirection of lowest pressure, most effective suction, whereby thestraight gas front will be distorted if subjected to a nonhomogeneoussuction. Moreover, a homogeneous suction effect will rectify a gas frontdistorted due to other possible reasons.

After the exit of the vapor-phase reactants from the reaction chamberand particularly the reaction space, their possible mixing how completewhatsoever will not cause harm to the thin film being grown.

Referring to FIG. 2, an embodiment slightly different from thatdescribed above is shown. The legend of reference numerals in thediagram is as follows:

-   21. Reaction chamber pack-   22. Inflow duct opening for starting materials of group A-   23. Inflow duct opening for starting materials of group B-   24. Connection for pipe communicating with suction inlet of pump-   25. Collecting outflow channel for outflow slits from in-parallel    stacked reaction chambers-   26. Suction groove encircling the planar element for collection of    possible gas leaks, whereby the suction groove communicates with the    collecting outflow channel.-   27. Intermediate plate serving for setting of outflow slit height,    that is, gas flow restriction. In addition to its function as    setting the outflow restriction, the intermediate plate forms a    baffle separating the different starting material groups at the    inflow side.-   28. Inflow channel for starting materials of group B-   29. Inflow channel for starting materials of group A-   30. Inflow channel serving for the distribution of the starting    material flows to the paralleled reaction chambers-   31. Top plate and one half of the uppermost reaction chamber-   32. 0-100 pcs. identical planar elements. Each planar element in    combination with the substrate forms the separating wall between two    superimposed reaction chambers as well as the inflow and outflow    channels in combination with the intermediate plates and the other    auxiliary plates.-   33. Bottom plate and one half of the lowermost reaction chamber-   34. Uppermost plate of support base-   35. Middle plate of support base-   36. Lowermost plate of support base-   37. Substrates-   38. Reaction chamber-   39. Restriction for gas flow leaving the substrate.

The embodiment shown in FIG. 2 is used in the same fashion as thatillustrated in FIG. 1. However, this construction differs from firstembodiment in that the starting materials of different reactant groupsare taken along their own inflow channels up to the inflow slits of thereaction chambers. Hence, the reaction chamber pack is assembled ontosuch a support base plate set 34-36 which after their stacking provideindividual flow ducts 22, 23 for the reactants of the different startingmaterial groups. Similarly, the reactants travel in their individualinflow channels 28, 29 at the side of the reaction chamber pack.

The gases are fed from the inflow channels 28, 29 separated by thebaffles 27 formed by the intermediate plates, whereby the height of thereaction chamber is dimensioned so that diffusion performs efficientintermixing of the flows entering from the different channels. Whilediffusion is a too slow mixing method to be used in the width directionof the flattened gas flow pattern, it performs well in the heightdirection. Thus, when the reactant is fed from one inflow channel 28,for instance, the inactive gas is fed from the other channel 29. Whenimpinging on the baffles, the reactant and inactive gas flows,respectively, are flattened assuming a planar flow pattern, whereby theyare homogenized during their intermixing in the inflow slit of thereaction chamber.

The inflow ducts 22, 23 and the inflow channels 28, 29 may have acircular cross section, for instance, and the reactant gas flows arespread into a fanned and flattened shape only at the baffles.

Analogously with the first embodiment, securing the equalizedcross-directional outflow pattern of the gas front is extremelyimportant.

Still referring to FIG. 2, it must be noted that the positions of theinflow channels 28 and 29 are slightly displaced for clarity. In apractical embodiment, these inflow channels are arranged in parallel,that is, adjacent in the lateral direction, whereby their infeedopenings into the reaction chamber will be located at the same distancefrom the substrate.

EXAMPLE

The following example describes the design principles for the pump ofthe apparatus shown in FIG. 1 and the interval between the successivevapor-phase reactant pulses, respectively, that make the apparatusperform in accordance with the invention:

Substrate size 300 × 300 mm² Number of substrates 10 pcs. Number ofreaction chambers 5 pcs. Spacing between substrates 4 mm Total volume ofreaction chambers 5 × 300 × 300 × 4 mm³ = 1,800 cm³ Dimensions/volume ofinflow 300 × 10 × 100 mm = 300 cm³ channels Dimensions/volume of outflow300 × 10 × 100 mm = 300 cm³ channels Total volume 2,400 cm³, or approx.2.4 L.

The pump capacity is selected as 360 m³/h, or 360×1000/3600 (l/s)=100l/s

Hence, the above-calculated total gas volume can be evacuated with apump so dimensioned in approx. 0.024 s.

A pump with the above-calculated capacity requires a pumping line withan inner diameter of 76 mm, having a volume per length unit ofπ×0.38×0.38×10 dm³=4.07 l/min, which means that if the length of thepumping line from the reaction chamber pack to the outlet connection ofthe apparatus is 1 m, for instance, its evacuation takes an extra timeof 0.04 s.

Accordingly, the interval between the reactant pulses in the aboveexample is selected as approx. 0.25 s, which is a sufficient time forone-time evacuation of the entire gas volume of the apparatus during theinterval between two successive reactant pulses. By extending theinterval between the pulses to 1 s, for instance, the total gas volumecan be evacuated approx. 4 times. Here, an inactive gas mayadvantageously be introduced to the reaction space during theevacuation.

We claim:
 1. A method of depositing a thin film on a substrate by atomiclayer epitaxy, in which method a substrate placed in a reaction spaceincluding a reaction chamber is subject to alternately repeated surfacereactions of at least first and second vapor-phase reactants for thepurpose of forming a thin film by surface controlled growth, thereaction space defining a reaction space volume, the method comprising:feeding the first and second reactants in the form of vapor-phase pulsesrepeatedly and alternately, each reactant fed from a separate source,into the reaction space in a plurality of deposition cycles, wherein thefirst reactant is fed through a first inflow channel that opens directlyinto the reaction space and the second reactant is fed through aseparate second inflow channel that opens directly into the reactionspace, and wherein each deposition cycle comprises: providing the firstreactant pulse through the first inflow channel while flowing only aninactive gas through the second inflow channel; stopping provision ofthe first reactant; removing first reactant from the reaction space;providing the second reactant pulse through the second inflow channelwhile flowing only the inactive gas through the first inflow channel;stopping provision of the second reactant; and removing second reactantfrom the reaction space; wherein the vapor-phase reactants react with asurface of the substrate for the purpose of forming a solid-state thinfilm compound on the substrate at a rate controlled by the surfacechemistry properties of the reactants, wherein each pulse forms no morethan a monolayer of reactant on the substrate, and wherein removingcomprises moving sufficient gas through the reaction space to removemolecules of first and second reactant adsorbed on inner walls of thereaction space in an interval between each two successive vapor-phasereactant pulses.
 2. The method of claim 1, wherein at least threereaction space volumes of gas are moved through the reaction space inthe interval between each two successive vapor-phase reactant pulses. 3.The method of claim 1, wherein 3-10 reaction space volumes of gas aremoved through the reaction space to evacuate the reaction space in theinterval between each two successive vapor-phase reactant pulses.
 4. Themethod of claim 1, wherein residual components of an immediatelypreceding vapor-phase reactant pulse remaining in the reaction chamberare reduced to a level of less than 1% during the interval prior to theinflow of a subsequent vapor-phase pulse.
 5. The method of claim 1,wherein residual components of an immediately preceding vapor-phasereactant pulse remaining in the reaction chamber are reduced to a levelof less than 0.1% during the interval prior to the inflow of asubsequent vapor-phase pulse.
 6. The method of claim 1, wherein removingcomprises feeding an inactive gas into the reaction space in theinterval between each two successive vapor-phase reactant pulses.
 7. Themethod of claim 6, wherein the reaction space is connected to a pumphaving a volumetric capacity which during the interval between each twosuccessive vapor-phase reactant pulses is appreciably greater than thereaction space volume.
 8. The method of claim 1, wherein the reactionspace is defined by the reaction chamber configured to house thesubstrate, further comprising one or more gas outflow channels for theoutflow of excess components of the reactant pulses from the reactionchamber, characterized in that at least a portion of the first andsecond inflow channels and gas outflow channels are provided with anarrow, oblong cross section in order to minimize the reaction spacevolume.
 9. The method of claim 1, wherein the reaction space is definedby the reaction chamber configured to house the substrate, and whereinthe reaction chamber is provided with a narrow, oblong cross section inorder to minimize the reaction space volume.
 10. The method of claim 9,characterized in that the first and second inflow channels have anarrow, oblong cross section in order to form at least essentiallyplanar pulses of vapor-phase reactant and to improve intermixing of thevapor-phase reactant flow with a carrier gas flow.
 11. The method ofclaim 9, characterized in that the vapor-phase pulses of each reactantgroup are fed via their individual inflow channels directly into thereaction chamber, wherein the vapor-phase pulse is allowed to intermixwith a carrier gas flow prior to bringing the reactant into contact withthe substrate.
 12. The method of claim 9, wherein the first and secondinflow channels each include an infeed opening to the reaction chamberhaving a width-to-height ratio between about 5:1 and 50:1.
 13. Themethod of claim 12, wherein the infeed opening width-to-height ratio isbetween about 5:1 and 30:1.
 14. The method of claim 1, wherein thevapor-phase reactant pulses are fed in a laminar flow into the reactionchamber.
 15. The method of claim 1, wherein the interval is on the orderof 1 second.
 16. The method of claim 15, wherein at least three reactionspace volumes of gas are moved through the reaction space in theinterval between each two successive vapor-phase reactant pulses. 17.The method of claim 15, wherein 3-10 reaction space volumes of gas aremoved through the reaction space in the interval between each twosuccessive vapor-phase reactant pulses.
 18. The method of claim 1,wherein feeding the reactants comprises defining a gas flow frontentering the reaction chamber with a width between about 10 cm and 30CM.
 19. The method of claim 18, wherein the gas flow front has a heightbetween about 1 cm and 3 cm.
 20. The method of claim 1, wherein thereaction space defines a width-to-height ratio between about 1:1 and100:1.
 21. The method of claim 20, wherein the width-to-height ratio isbetween about 5:1 and 50:1.
 22. The method of claim 20, wherein thewidth-to-height ratio is between about 5:1 and 30:1.